Semipermeable membrane support and process of preparation thereof

ABSTRACT

A semipermeable membrane support includes, a non-woven fabric including main fiber and binder fiber, both of which are formed of synthetic resin fine fiber, the non-woven fabric being manufactured by heating and pressing after paper making, wherein the semipermeable membrane support has a ratio of a tensile strength in a paper feeding direction to that in a width direction of 2:1 to 1:1.

FIELD OF INVENTION

[0001] The present invention relates to a semipermeable membrane supportand a process of making the same.

DESCRIPTION OF THE RELATED ART

[0002] Conventionally, the support having a laminated double layeredstructure which includes a high density non-woven fabric and a lowdensity non-woven fabric has been proposed as a semipermeable membranesupport (Japanese Patent Laid-Open Publication No. Shou 60-238,103 andJapanese Patent Examined Publication No. Hei 5-35009). On the otherhand, the support having a single layer structure has been also proposedso as to simplify the manufacturing method and lower production costs.(Japanese Patent Laid-Open Publication No. Hei 10-225630)

[0003] However, it is difficult to expect the similar effect in thesingle layer structure to that in the double layered structure. On theother hand, the cost of manufacturing the support having a multi-layeredstructure including a double layered structure is high. Therefore, interm of costs, the manufacturing method is desired wherein apredetermined difference in surface roughness between the front surfaceand the rear surface is provided in the single layer structure to ensurea prescribed filtration resistance. However, when the (polymer) castingsolution is applied to one face of the support, the support bends in thewidth direction, which becomes a hindrance to the treatment in thecoagulation and rinsing bath following the roll feed, resulting in anuneven semipermeable membrane produced.

[0004] The inventors directed their attention to the fact that such acause can be reduced by setting suitable conditions in the paper makingand have studied wholeheartedly. The inventors found that a main causeis in that the ratio of the tensile strength of the support in the paperfeeding direction to that in the width direction is more than 2:1, andthat when the (polymer) casting solution is applied to the support, thesupport bends in the width direction and becomes an obstacle in theprocess within the coagulation and rinsing bath after roll feeding. Thebend of the support caused by the application of the semipermiablemembrane solution to the support having a large tensile strength ratiocan be reduced to some extent by increasing the weight of the support.But, in such a case, the cost is high and the thickness of the supportis large, resulting in problem of the membrane area being reduced withina certain volume.

SUMMARY OF THE INVENTION

[0005] The present invention has been developed to overcome theabove-described disadvantages.

[0006] It is accordingly an objective of the present invention toprovide a support and a process of producing the same without anobstacle to the manufacturing of a semipermeable membrane from the basepaper.

[0007] According to the present invention, there is provided asemipermeable membrane support of the present invention includes anon-woven fabric including main fiber and binder fiber, both of whichare formed of synthetic resin fine fiber, the non-woven fabric beingmanufactured by heating and pressing after paper making, wherein thesemipermeable membrane support has a ratio of a tensile strength in apaper feeding direction to that in a width direction of 2:1 to 1:1, toprevent the bend in the width direction during the semipermeablemembrane formation.

[0008] According to the present invention, there is provided asemipermeable membrane support which includes a non-woven fabric formedfrom a main fiber and a binder fiber made of polyester fiber having anaverage single fiber fineness of 0.6 to 8.9 decitex and manufactured bytreating in the thermal pressing process after paper making, wherein thesupport has an air permeability of 0.5 to 7.0 cc/cm²/sec and an averagepore size of 5 to 15 μm as well as a ratio of a tensile strength in thepaper feeding direction to that in a width direction of not more than2:1, to restrict the bend thereof to within the tolerable range duringthe roller feeding in the semipermeable membrane formation, resulting inincrease of the quality precision. More preferably, the ratio of thetensile strength in the paper feeding direction to that in the widthdirection may be not more than 1.5:1.

[0009] When the semipermeable membrane support has an air permeabilityof not more than 0.5 cc/cm²/sec, or has an average pore size of not morethan 5 μm, the penetration of the (polymer) casting solution into thesemipermeable membrane support is disturbed and thereby, the problem ofdecrease of adhesive strength between the semipermeable membrane and thesemipermeable membrane support due to let go anchor effect is likely tocome up. On the contrary, when the semipermeable membrane support has anair permeability of not less than 7.0 cc/cm²/sec, or has an average poresize of not less than 15 μm, the penetration of the (polymer) castingsolution into the semipermeable membrane support is too much andthereby, the problem of partial over-penetration of the (polymer)casting solution to the back surface is likely to come up.

[0010] A method of manufacturing a semipermeable membrane support of thepresent invention includes the steps of:

[0011] preparing a dispersed and mixed solution by dispersing and mixingin a solution main fiber and binder fiber, both of which are formed ofpolyester fiber having an average single fiber fineness of 0.6 to 8.9decitex, in a mixing ratio of 20:80 to 70:30;

[0012] making paper from the dispersed and mixed solution whilecontrollering a flow rate of the dispersed and mixed solution so thatthe semipermeable membrane support has a ratio of tensile strength in apaper feeding direction to that in a width direction of 2:1 to 1:1; and

[0013] heating and pressing the paper so that the semipermeable membranesupport has a surface roughness of a front surface is larger than thatof a rear surface by 15% or more after drying.

[0014] Using the above-mentioned support, that is, the semipermeablemembrane support which includes a non-woven fabric including a mainfiber and a binder fiber formed from synthetic resin fine fibers havingan average single fiber fineness of 0.6 to 8.9 decitex and manufacturedby treating in the thermal pressing process after paper making, whereinthe support has a ratio of a tensile strength in the paper feedingdirection to that in the width direction of 2:1 to 1:1, even when apolymer solution is applied to at least one surface of the support, thetreatment in the coagulation and rinsing bath without any obstacles canbe achieved after roll feeding with no bend in the width direction,resulting in formation of a semipermeable membrane with excellentquality. Preferably, the support has an air permeability ot 0.5 to 7.0cc/cm²/sec and an average pore size of 5 to 15 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The above and other objectives and features of the presentinvention will become more apparent from the following description of apreferred embodiment thereof with reference to accompanying drawings,throughout which like parts are designated by like reference numerals,and wherein:

[0016]FIG. 1 shows a process drawing according to the present invention.

[0017]FIGS. 2A and 2B show microphotographs of the supports (magnifiedby 200 times) made by means of the method of the present invention andthe conventional method, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] This application is based on application No.2000-289409 filed inJapan, the content of which is incorporated herein by reference.

[0019] The present invention will be described in detail with referenceto FIG. 1 showing an embodiment of the present invention.

[0020] The process for making the support of the present inventioncomprises (A) making paper and (B) processing the resulting paper. Theprocess for making a semipermeable membrane such as a reverse osmoticmembrane comprises (C) making a semipermeable membrane using theresulting support and (D) forming a skin layer on the semipermeablemembrane to obtain a final RO membrane. Main fiber constructing a basicstructure in the non-woven fabric which is used in the presentinvention.

[0021] A main fiber made of synthetic resin fiber is preferablypolyester fiber having an average single fiber fineness of 0.6 to 8.9decitex, more preferably 0.6 to 6.7 decitex. If the average single fiberfineness is not more than 0.6 decitex, the semipermeable membranesupport is likely to have an air permeability of not more than 0.5cc/cm²/sec and an average pore size of not more than 5 μm. On thecontrary, If the average single fiber fineness is not less than 8.9decitex, the semipermeable membrane support is likely to have an airpermeability of not less than 7.00 cc/cm²/sec and an average pore sizeof not less than 15 μm.

[0022] Binder fiber used in the present invention

[0023] The non-woven fabric for the semipermeable membrane support maybe preferably made by mixing the main fiber and the binder fiber. Thenon-woven fabric may be made without the binder fiber mixed. If thenon-woven fabric is made with the binder fiber mixed, the resultingnon-woven fabric may have an enhanced strength due to the weld of thefiber at the intersection point by the binder fiber during thermalpressing process, and a smooth surface, resulting in a more preferablefabric for the semipermeable membrane support. The binder fiber mayinclude a thermoplastic fiber such as polyester fiber, polyolefin fiber,nylon fiber, alamide fiber, polyphenylene sulfide fiber and the like. Ofthem, the polyester-based binder fiber may be most preferable,comprehensively considered in term of mechanical strength, thermalprocess characteristics and cost.

[0024] The polyester-based binder fiber may include a low melting pointpolyester fiber, an undrawn polyester fiber and so on. The melting pointor glass transition point of the polyester based binder fiber is lowerthan that of the main fiber and preferably within the range of 120 to260° C.

[0025] The mixing amount of the binder fiber may be preferablydetermined on the basis of the binding degree of the main fibers. Theabove-mentioned amount may be preferably 20 to 80% by weight, morepreferably 30 to 70% by weight. If the mixing amount of the binder fiberis too small, the resulting fabric is likely to have a low strength anduneven surface. If the mixing amount of the binder fiber is too large,the cost is high and the desired air permeability is less liable to beachieved.

[0026] Process of dispersion and mixing

[0027] The non-woven fabric is made in the above-mentioned mixing ratioaccording some manufacturing process such as dry method, wet method andso on. However, the wet method may be preferably used, because whollyeven non-woven fabric can be obtained. According to the wet method,first, the main fiber and the binder fiber are dispersed into waterevenly, and then passed through the process such as a screening process(for example, for removal of foreign matters, agglomerates and thelike), so as to adjust the final fiber concentration to 0.01 to 0.50% byweight. And then, the paper is made.

[0028] Some reagents such as a co-dispersant, defoaming agent,hydrophilic agent or anti-static agent may be added to obtain more evennon-woven fabric.

[0029] The non-woven fabric is made as one sheet with a paper makingmachine according to the wet method. Some sheets may be made accordingto the wet method and then laminated. The non-woven fabric made bylaminating some sheets according to the wet method is more even.

[0030] Process for making paper

[0031] The support of the present invention has a ratio of a tensilestrength in the paper feeding direction to that in the width directionof 2:1 to 1:1. Therefore, the adjustment must be performed in theprocess of making paper to achieve the above-mentioned ratio. It issuitable to make paper on the inclined wire cloth so as to adjust thetensile strength ratio to the above-mentioned range.

[0032] The tensile strength ratio can be adjusted to the above-mentionedrange by controllering the concentration of the material dispersionmixed solution, flow velocity, the rate of the wire in the inclined wirecloth, the inclination angle and so on.

[0033] Thermal pressing process

[0034] The resulting fabric is heated and pressed, resulting in asemipermeable membrane support. The thermal pressing process will bedescribed in the following part, but the present invention is notlimited to the below-mentioned process.

[0035] The fabric is transported in the state pinched between tworollers and heated and pressed continuously. Both or either one of tworollers is used as a heat roller. The degree of thermal pressing iscontrolled by adjusting the surface temperature of the heat roller, thepinching force of the rollers, and the transporting velocity of thenon-woven fabric, or time of pressing the fabric. The surfacetemperature of the roller is preferably 150 to 260° C., more preferably200 to 240° C. The pinching force of the rollers is preferably 20 to 180kg/cm, more preferably 40 to 150 kg/cm. The transporting velocity of thenon-woven fabric is preferably 10 to 100 m/min, more preferably 20 to 60m/min. The non-woven fabric to be transported may be a single layer,layered structure made of several layers of one kind, or layeredstructure of several layers of some kinds. The non-woven fabric whichhas been heated and pressed and the non-woven fabric which has not beenyet processed may be laminated and bonded by means of the thermalpressing process.

[0036] The surface temperature of the heat roller, the pinching force ofthe rollers and the transporting velocity of the non-woven fabric areadjusted on the basis of the desired characteristics of thesemipermeable membrane support. The process conditions of the front andrear surfaces must be changed to make central line average roughness ofthe front and rear surfaces of the semipermeable membrane support to bedifferent by not less than 15%. When the surface temperature of the heatroller and the pinching force of the roller are high and thetransporting velocity of the non-woven fabric is slow, large amount ofheat is absorbed. On the contrary, when the surface temperature of theheat roller and the pinching force of the roller are low and thetransporting velocity of the non-woven fabric is fast, small amount ofheat is absorbed.

[0037] The above-mentioned conditions and the size and mixing ratio ofthe material fibers to be used in the paper making process arecontrollered well, so as to obtain a semipermeable membrane supporthaving a different central line average roughness between the front andrear surfaces by not less than 15%.

[0038] Process for applying the (polymer) casting solution

[0039] One example of the process in which the (polymer) castingsolution is applied to one surface of the semipermeable membrane supportof the present invention will be described.

[0040] As in the membrane forming process (c), the support istransported along the drum (roller) while the (polymer) casting solutionis applied to the upper surface of the support in the state of membrane.The (polymer) casting solution is filled in the hopper located above thedrum (roller). The hopper has a lower end close to the surface of thesupport, so as to prevent the (polymer) casting solution from leakingthrough the slit between the hopper and the support.

[0041] The support is introduced into the coagulation bath along thedrum (roller) so as to solidify the (polymer) casting solution that hasbeen applied to the support. When the support is introduced into thecoagulation bath, the support leaves the drum (roller) and is immersedin the bath.

[0042] The polymer solution may be prepared by dissolving, for example,polysulfone at the concentration of about 16.5% by weight intoN,N-dimethylformamide (DMF). When this polymer solution contacts withwater, polysulfone is coagulated and solidified. Therefore, such apolymer solution is immersed into the coagulation bath including waterand coagulated. Then, the support passing through the coagulation bathis immersed into the rinsing bath to remove the remaining DMF, andfurther to be solidified. Thus, the polysulfone layer of 20 to 100 μm isformed on the surface of the support.

[0043] If the ratio of the tensile strength in the paper feedingdirection to that in the width direction of the support is large duringthis process, the support is likely to bend in the width directionduring formation of the semipermeable membrane, resulting in the failureof the transporatation of the the support with a roller. According tothe present invention, the ratio of the tensile strength of the supportin the paper feeding direction to that in the width direction isadjusted to the range of 2:1 to 1:1 and therefore, the bend of thesupport can be controllered within the tolerable range with no hindranceto the roller transportation.

[0044] Thereafter, the active layer (skin layer) is coated on thesurface of the semipermeable membrane. The active layer is made ofcellulose such as cellulose acetate, polyamides, polyimides and the likedepending on its application. The active layer has a smaller thicknessthan the semipermeable membrane.

[0045] Generally, the semipermeable membrane on which the active layerhas not been yet coated is called a microfiltration membrane orultrafiltration membrane. While, the semipermeable membrane on which theactive layer has been already coated is called a nano-filtrationmembrane or reverse osmosis membrane.

[0046] The support of the present invention finds extensive applicationin such wide fields as desalination and water reuse, dairy, foods,remedy, chemistry, nuclear engineering, coloring process engineering,and the like, and can be used as a support for a respectivesemipermeable membrane.

[0047] The present invention will be described in details in thefollowing examples, but is not limited thereto. The properties in theexamples were obtained in the following way. In the following examples,% means % by weight, unless otherwise specified.

[0048] Weight was determined on the basis of JIS P 8124.

[0049] Thickness was determined on the basis of JIS P 8118.

[0050] Ratio of tensile strength was determined by measuring the tensilestrengths in the longitudinal direction (paper feeding direction) and inthe transverse direction (paper width direction) on the basis of JIS P8113 and then, calculating the strength in the longitudinaldirection/that in the transverse direction.

[0051] Air permeability was measured with a Fragile-type testing machineon the basis of JIS L 1096.

[0052] Surface roughness was determined on the basis of JIS B 0601.

[0053] Pore size was determined on the basis of the bubble point method(ASTM F316-86, JIS K 3832). The pore size (average pore diameter) in thesupport is an index of whether there are many small pores or not, whenthe air permeability is at the same level.

EXAMPLE 1

[0054] 60% of drawn polyethylene terephthalate (PET) fibers having anaverage single fiber fineness of 1.7 decitex and 3.3 decitex and a fiberlength of 5 mm and 40% of undrawn PET fibers having an average singlefiber fineness of 1.2 decitex and a fiber length 5 mm were dispersedfully in the chest to prepare an aquous slurry having a fiberconcentration of 0.05%. The resulting slurry was transported to theinclined wire cloth paper making machine and a non-woven fabric,three-dimensional assembly of the fibers, was made while the ratio oftensile strength in the paper feeding direction to that in the widthdirection was controllered (weight: 68 g/m²).

[0055] Both faces of the resulting non-woven fabric were processed underthe conditions that temperature was 225° C., the pressure was 60 kg/cm,and the speed was 25 m/min, with a calender comprising a combination ofa heating metallic roller and elastic roller.

[0056] The properties of the resulting support are shown in table 1.

COMPARATIVE EXAMPLE 1

[0057] The aqueous slurry was prepared using the same mixing ratio andin the same way as in Example 1. Using the resulting slurry, paper wasmade with a cylinder machine (weight: 68 g/m²).

[0058] Calendering was performed under the same conditions as in Example1.

COMPARATIVE EXAMPLE 2

[0059] A paper making web A′ (weight: 34 g/m²) was made with a cylindermachine using the same mixing ratio and in the same way as in Example 1.

[0060] Next, a paper making web B (weight: 34 g/m²) was made with acylinder machine from 60% of drawn PET fibers having an average singlefiber fineness of 3.3 decitex and a length of 5 mm and 40% of undrawnPET fibers having an average single fiber fineness of 1.2 decitex and alength 5 mm under the same conditions as in Example 1.

[0061] The paper making web A′ was processed with the similar calenderto that in Example 1, under the conditions that temperature was 220° C.,the pressure was 100 kg/cm, and the speed was 50 m/min.

[0062] Then, the paper making web B and the paper making web A′ asprocessed in the above-mentioned way were laminated and processed underthe conditions that temperature was 230° C., the pressure was 130 kg/cm,and the speed was 40 m/min. TABLE 1 Comparative Comparative Example 1Example 1 Example 2 Weight 70 70 71 (g/m²) Thickness 96 94 87 (μm)Tensile strength 1.3 4.2 4.4 ratio Longitudinal/ transverse Airpermeability 3.4 3.1 5.0 (cc/cm²/sec) Difference in 33.4 28.6 14.7surface roughness (%) Pore size 10.1 10.3 12.1 Average value (μ)

[0063] The (polymer) casting solution was applied to the supportsobtained according to Example 1, Comparative Examples 1 and 2 in themembrane making process. In Example 1, the bend of the support in thewidth direction was a little and the formed membrane was even and hadgood membrane properties.

[0064] On the contrary, in Comparative Examples 1 and 2, the bend of thesupport in the width direction was large and thereby, the support couldnot pass through the process smoothly, resulting in occurrence ofwrinkles.

[0065]FIGS. 2A and 2B show microphotographs of the sample in Example 1,in which the ratio of the tensile strength in the paper feedingdirection to that in the width direction is small, and of the sample inComparative Example 2, in which conventionally, the ratio of the tensilestrength is large, by an electron microscope, respectively.

[0066] These microphotographs apparently show that the orientation ofthe fibers is different between the two samples. As shown in FIG. 2B,since the fibers are orientated in the longitudinal direction, thetensile strength ratio is large and the bend in the width direction ofthe support is large when the (polymer) casting solution is applied tothe support. On the contrary, as shown in FIG. 2A, since the fibers arein a random orientation to some extent, the tensile strength ratio issmall and the bend in the width direction of the support is small evenwhen the (polymer) casting solution is applied to the support. Thebinder fibers lie between the main fibers and bind them due to thethermal transformation.

[0067] As mentioned above, the present invention provides asemipermeable membrane support and a process for making the same, inwhich the semipermeable membrane support in the single layer structurehaving the same effects as those of the two-layered structure can beachieved at low costs without a hindrance in the production of thesemipermeable membrane from the base paper. Conventionally, an attemptto realize a desired filtration resistance has been made by setting acertain difference in surface roughness between the front and rearsurfaces of the support in a single layer structure. However, since thesupport bent in the width direction when the (polymer) casting solutionwas applied to one surface of the support, the processing of the supportin the coagulation and rinsing bath after roll transportation washindered, resulting in formation of an uneven semipermeable membrane.According to the present invention, the semipermeable membrane supporthaving a ration of the tensile strength in the paper feeding directionto that in the width direction of 2:1 to 1:1 is used, so as to solve theabove-mentioned problems. The semipermeable support in such a structurehas a property of preventing the bend in the width direction. Even whenthe (polymer) casting solution is applied to one surface of the support,the roll transportation and processing in the coagulation and rinsingbaths of the support can be achieved without the bend in the widthdirection, with the result that the semipermeable membrane having anexcellent quality can be made.

[0068] Although the present invention has been fully described by way ofexamples with reference to the accompanying drawings, it is to be notedhere that various changes and modifications will be apparent to thoseskills in the art. Therefore, unless such changes and modificationsotherwise depart from the spirit and scope of the present invention,they should be constructed as being included therein.

What is claimed is:
 1. A semipermeable membrane support comprising, anon-woven fabric including main fiber and binder fiber, both of whichare formed of synthetic resin fine fiber, the non-woven fabric beingmanufactured by heating and pressing after paper making, wherein thesemipermeable membrane support has a ratio of a tensile strength in apaper feeding direction to that in a width direction of 2:1 to 1:1.
 2. Asemipermeable membrane support according to claim 1, wherein the mainfiber is formed of polyester fiber having an average single fiberfineness of 0.6 to 8.9 decitex.
 3. A semipermeable membrane supportaccording to claim 1, wherein the binder fiber is formed of polyesterfiber having an average single fiber fineness of 0.6 to 8.9 decitex. 4.A semipermeable membrane support according to claim 1, wherein thenon-woven fabric has an air permeability of 0.5 to 7.0 cc/cm²/sec.
 5. Asemipermeable membrane support according to claim 1, wherein thenon-woven fabric has an average pore size of 5 to 15 μm.
 6. Asemipermeable membrane support according to claim 1, wherein thesemipermeable membrane support has a capability of preventing from beingbent in the width direction during a manufacture of the semipermeablemembrane.
 7. A semipermeable membrane support according to claim 6,wherein the semipermeable membrane support has the ratio of the tensilestrength in the paper feeding direction to that in the width directionof 1.5:1 to 1:1.
 8. A semipermeable membrane support according to claim1, wherein a central line average roughness of a front surface of thesemipermeable membrane support is larger than that of a rear surface ofthe semipermeable membrane support by 15% or more, and wherein thesemipermeable membrane support has an anchor effect to a semipermeablemembrane when the semipermeable membrane support is applied with thesemipermeable membrane on the front surface thereof.
 9. A method ofmanufacturing a semipermeable membrane support comprising the steps of:preparing a dispersed and mixed solution by dispersing and mixing in asolution main fiber and binder fiber, both of which are formed ofpolyester fiber having an average single fiber fineness of 0.6 to 8.9decitex, in a mixing ratio of 20:80 to 70:30; making paper from thedispersed and mixed solution while controllering a flow rate of thedispersed and mixed solution so that the semipermeable membrane supporthas a ratio of tensile strength in a paper feeding direction to that ina width direction of 2:1 to 1:1; and heating and pressing the paper sothat the semipermeable membrane support has a surface roughness of afront surface is larger than that of a rear surface by 15% or more afterdrying.
 10. A method of manufacturing a semipermeable membrane supportaccording to claim 9, wherein the step of making paper is performed byusing an inclined wire cloth machine.
 11. A method of manufacturing asemipermeable membrane including a non-woven fabric containing mainfiber and binder fiber, both of which are formed of synthetic resin finefibers having an average single fiber fineness of 0.6 to 8.9 decitex,the non-woven fabric being manufactured by heating and pressing afterpaper making, said method comprising the steps of: applying a polymersolution to at least one surface of the semipermeable membrane supportwhich has a ratio of a tensile strength in a paper feeding direction tothat in a width direction of 2:1 to 1:1; roll feeding the semipermeablemembrane support to a coagulation and rinsing bath so that thesemipermeable membrane support is not bent in a width direction; andcoagulating and rinsing the semipermeable membrane support in thecoagulation and rinsing bath.